1. Field of the Invention
The invention relates to a safety paper having a structure making possible a contactless checking of an authenticity feature and to a method for checking the authenticity of documents which are recorded in an optically readable form on the safety paper.
2. Description of the Related Art
The use of such safety papers for producing documents serves for protecting them against unauthorized reproduction by forgers. This is necessary, in particular, for securities such as bank notes, checks, travelers checks, stocks, etc. There is also a need for securing papers which do not have a direct monetary value, such as identification papers, passports etc., against unauthorized copying. The employed term “document” is therefore meant to include all types of securities and identification papers to be protected against unauthorized copying.
In particular, in the case of securities which are circulated daily, for example, bank notes, a forger may succeed in copying the optically recorded document contents, for example, the optical printed image of the bank notes, in a deceptively precise way. A protection against this is the authenticity feature contained in the safety paper, used for producing the documents, as a result of the structure imparted to the safety paper during manufacture which authenticity feature supposedly practically cannot be copied by a forger with the means available to him. Moreover, the application of watermarks or the introduction of a safety thread into the paper is known. In particular, it is known (DE 29 05 441 C3) to introduce into the paper layer a magnetizable or electrically conducting safety strip. These conventional measures, however, can no longer be considered satisfactory in view of the advances of the working means employed by forgers. In particular, in the case of global political crisis regions the war-conducting groups or even entire countries employ forgery as warfare. Accordingly, the resources employed for forgery are correspondingly great.
In a bank note produced from a known safety paper of the aforementioned kind (DE 196 30 648 A1), a transponder chip with an antenna is integrated into the bank note in a way similar to that of safety strips of different kinds integrated into conventional bank notes. The pattern which serves as the antenna has the shape of an antenna coil which is formed as a thick or thin layer structure directly on the transponder chip. As an alternative, the transponder chip can be localized in the neighborhood of the metal safety strip or at an interrupted portion of the metal safety strip and can be surrounded by the antenna coil. According to a further possibility, the metal safety strip is modified such that it forms itself the antenna coil for the transponder chip. The attachment of the antenna coil on the transponder chip itself results in a very small coil cross-section with correspondingly small sending and receiving sensitivity. Moreover, it is difficult with regard to manufacturing technology to anchor the small transponder chip on the bank note. The other aforementioned options, in which the antenna coil is arranged outside of the transponder chip on the bank note, provide the possibility of forming greater coil cross-section but the formation and connection of the antenna coil to the transponder chip on the bank note is problematic with regard to manufacturing technology.